Method of producing chromate



Patented Feb. 25, 1947 METHOD OF PRODUCING CHROMATE Marvin J. Udy, Niagara Falls, N. ,Y.

i No Drawing. Application July5, 1941,

Serial No. 401,298

11 Claims. 1

This invention "relates to chromium recovery andhas for an object the provision of an improved method for recovering chromium from crude chromium-bearing material such as chromite ore. Moreparticularlp-the invention contemplatesthe provision of an improved method or process for "recovering chromium in chromate form from chromium-bearing A material containing chromium in theiforrn ofchromite. A further object of the-invention: is to'provide certain improvements in methods "or processes for producing chromate compounds.

According to some heretofore customary practices, chromate compounds are produced by subjecting chromite-bearing materials such as natural chromite oreand ,chromite'ore concentrates to .oxidation treatmentsv in the presenceof one or more basic substances such aslime and alkali metal compound. Thus, for. example, alkali metal chromates .may .be ,formedv by subjecting charges comprising finely divided chromite-bearing material and. an alkalimetal compoundsuch as sodium carbonate or sodium sulphate toan oxidizing or roasting treatment at a temperature in the range 750 C. to 1200 C. The product of the oxidizing or roasting treatment may be leached or digested with a solvent forthe alkali metal chromate produced ;to form a solution of the chromate,-and the solution may be separated from the insoluble material by filtration.

In order to accomplish effectiveoxidation, it-is much as three times the weight: of'thechromite ore. In the treatment of such chargeszatthe high temperaturesrequired for 'oxidation, even with the large amounts of diluent material included, it is difficult to avoid fusionwithconsequent reduction in the efficiency of oxidation,

and, even under optimum conditions of opera.-

tion, it-is difficult or impossibleto approachor accomplish complete oxidation. ,Satisfactory oxidation usually requires-heating:of the charge under oxidizing conditions; for manyhours.

Fusion of the roasting charge, in some cases, may be attributed to some extent to-the, presence in the charge of thelarge amount of relatively low-melting point alkali metal compound required to form alkali metal chromate with all of the chromium of the-chromite ore and to the production -of a.relatively large amount-of lowmelting point alkali metal chromate in such cases. Fusion maybe-avoided to alarge extent by employing the-alkali metal compound in-an amount substantially smaller. than that required to form alkali metal .chromate' .with all of the chromium-of the chromiteore and thus -avoiding. theinclusion and production oflarge amounts of relativelylow-melting. point-1 compounds. Ineffective or incompleteoxidation may be attributed to the exclusion ofltheloxygen ofthe air from contact withthe chromite particles resulting from agglomeration. of thecharge .particles orv coating of the particles with-alkali metal chromate produced .in the roasting treatment.

In employing rotary kilns and. hearth .furnaces provided with rabbling means in accordance with heretofore customary procedures, conversion to chromate of about eightypercent (80,%)-oi the chromiumpf the chromite oreofa roasting or oxidizing charge can be accomplished quite efiiciently, but higher degrees of. conversionare difficult and relatively expensive to accomplish.

The present invention provides improvements by means of'whichyless dilute oxidation. charges, or charges more concentrated with respectto chromium content, maybe employed to efiect improved recoveries of chromium in the 'form of chromate in shorter periods of time and with greater ease of handling and greater economy resulting from the use of the more concentrated charges.

The invention is based in part on my observation that oxidation of the chromium Of chro mite-bearing material proceeds rapidly during theearly stages of the oxidizing treatment and-on my discovery that, at the conclusion of the oxidizing treatment, the unoxidized chromium still remains as a constituent of undecomposed chromite of the charge subjectedito oxidation. I have :observed, for example, that in a :given charge,'containing lime, chromiteand an alkali 1 metal compound such'as sodium carbonate, in

which a maximum'of'about ninety percent (90%) conversion to chromatecan'be efiected in twelve to fourteen hours, a conversion of as much as eighty percent, is effected 'within about threerhours or less.

I have found that about the same amountof chromium will be oxidized in the initial stages of the oxidation treatment whether the amount of alkali metal compound employed is sufi'icient to form alkali metal chromate with all of the which haveapparent specific gravities lowe r than. the specific gravity of chromite mineral remaining undecomposed with the result thatthe undecomposed chromite mineral may be separated from the solid materials associated therewith by gravity concentration methodssuch, for example, as tabling and water classification. Compounds such as iron oxide, silica, alumina and magnesia initially associated with that portion of the chromite mineral which is decomposed in the oxidation treatment are converted in whole or inpart into compounds of sufiiciently low specific'gravities or of such physical forms that theymay be floated 'or washed away from the particles of undecomposed chromite mineral by means of 1, water. 'My investigations haveaindicated that iron is 'theimost difilcult .of the original. constit'-- uents of thechromite mineral to separate from the undecomposed chromite mineral. The chro- 1 -mite mineral recovered. by concentration may be contaminated to some extent with iron in the form of ferric oxide. ,If desired, this contaminating iron may be removed by treating the concentrate with an acid such as'sulphuric acid'in aqueous solution. I I

According to the present invention, chromiumbearing material such as chromite ore is subjected to an oxidizing treatment under condltions such as to convert chromium contained therein chromium or with only about seventy to eighty 5 to the chromate condition. Chromate produced percent (70 to 80%) of the chromium. in the oxidation treatment is separated or re- According to the present invention, the tendcovered from the oxidation charge (comprising ency of the charge to oxidize effectively with a chromate, undecomposed chromlte and solid de limited amount of alkali metal compound is composition products containing the iron ox1de, taken advantage of to provide an oxidation silica, magnesia and alumin of the chromite charge (containing a limited amount of alkali V V subjected to oxidation) by digesting or leachmg metal compound) which sinters or fuses less" the oxidation charge with a liquid solvent for the readily under the conditions employed in the chromate. The chromate solution is separated oxidation treatment, and the fact that theunfr m th nsol m teri ls of the char e y oxidized chromium remains a a constituent f sedimentation O1 filtration, and the insoluble the original chromite is taken advantage of to, residue is subjected to a concentrationtreatment recover the unoxidized chromium by mean'sfofial ,to recover undecomposed chromite. concentration treatment. The use of a relatively The chromite ore subjected to oxidation may small amount of alkali metal compoundin the be natural chromite ore or altered or modified oxidation charge also permits the use of smaller C o te or p for p y hoatlng amountsof diluent materials such as lime, and natural C om t ore preliminarily With m to charges containing limited amounts of diluent effect alteration of the chromite mineral of the material and alkali metal compound may be ore. Heating of the ore with lime may be carried oxidized at higher temperatureswith less danger out in accordance with procedures outlined in of undesirable sintering, or fusing resulting and y oopending pp t Se ial No. 25 ,7 with consequent increase in the rate of oxidation filed January 1939, a Serial N of the chromium. The components of the charge filed August 3, 1940- Heating of the ore W may also be employed in coarser states of subo cts a teration of v t e Ore and the prodivision. Thus, the use of a limited amount of duction of a p t nta ning an altered chroalkali metal compound permit the use of Submite by substitution of the lime for ferrous oxide stantially more concentrated and' coarser oxidaof c o t o the i i l ore and makes the tion charges which facilitates subsequentleach- Chromium of the more y OXidiZable t ing and filtering operations and improves the the chromate condition. In efiecting alteration efficiency of the oxidation treatment by reducing of the ore, a charge comprising natural chromite the time of oxidation. Charges consisting largely ore and lime or limestone in intimate admixture. of particles small enough to pass a IOU-mesh may be heated, for example, to a fusing temperscreen can be treated substantially as efficiently ature. Fusion may be of the type in which a in the process of the invention as charges conmolten glassy product is formed or it may be of sisting largely of much smaller particles can be the type in which a sintered product or a fritted treated in processes of the type employed heretoproduct is formed. Temperatures in;the range fore. The invention permits the use of smaller 1200 C. to 1350 C. may be employed to produce charges than those employed in heretofore cussintered or fritted products. Higher temperatomary processes to produce the same amounts tures are required to .produce molten products. of chromate; I 3 Any desired amount of lime or limestone rela- Oxidation of thechromite ore in the presence tively to chromite ore may be employed in a of basic materials. such as lime and alkali meta-1 charge treated to effect alteration of chromite Compounds rosultoin decomposition a large ore. For example, lime or limestone may be emproportion of the chromite mineral of the charge ployed in amount sufiicient to provide one mole- W t production o decomposition products cule of calcium oxide for each molecule of the acid components or constituents of the ore, or lime or limestone may be employed in amount sufficient to provide more than one molecule or several molecules of calcium oxide for each molecule of the acid constituents or'components of the ore. The components of a charge to be submetal compound to oxidize chromium of the chro mite to the hexavalent state with the production of chromate, treatment of the oxidized charge to separate chromate from the acccmpanying material, and a concentration treatment of the olid residue remaining after separation of the chromate'to separate and recover chromite of the original charge containing the unoxidi'zed chromium. The concentrate recovered may be reintroduced into the oxidationfurn'ace with ad- Intimate mixing can be accomplished ditional chromite or it may be accumulated and treated separately.

The charge subjected to roasting or oxidation preferably comprises chromite, lime in amount equal to about one and one-half to two times the weight of the chromite and sodium carbonate in amount sufficient to form sodium chromate with about seventy to eighty percent ('70 to 80%) of the chromium of the chromite. The materials of the charge preferably are intimately mixed together and in the form of particles small enough to pass a, IOU-mesh screen. Mixing may be accomplished effectively by grinding together the components of the charge.

Roasting or oxidation preferably is carried out with air in a rabble furnace at a temperature in the range 750 C. to 1000 C. for a period of'about three hours.

Leaching preferably is carried out with an aqueous liquid containing some sodium carbonate in solution. Maintenance of some sodium carbonate in the leaching liquor insures the recovery as sodium chromate of any calcium chromate formed in. the course of the oxidizing treatment. The leaching solution may be treated in any suitable manner for the recovery'in suitable form of the chromium contained therein as sodium chromate.

The solid insoluble residue remaining after separation of the soluble chromate is subjected to a water classification or concentration treatment, preferably in the form of a table concentration operation in which the undecomposed chromite is separated by gravity from the lighter decompo sition products associated therewith. The concentration treatment results in the recovery in the form of a low-volume concentrate of ninety percent (90%) or more of the undecomposed chromite. The chromium content of the original or rough concentrate is only slightly lower than the chromium content of the original chromite subjected to oxidation. A clean concentrate containing about the same chromium content a that of the original chromite may be produced by treating the rough concentrate with an acid such as sulphuric acid to remove iron oxide associated with the chromite particles.

By concentrating the residue remainin after separation of the soluble chromate from the oxi dation charge and returning the concentrate to the process for oxidation to chromate an over-all recovery of chromium as chromate in excess of ninety percent (90%) can be accomplished consistently.

I claim: v

1. The method of producing chromate which comprises roasting a chargeof chromite-bearing material, lime and soda ash while in intimate contact with air at a temperature sufficiently high to cause decomposition of the, chromite and oxidation of the chromium thereof to the hexavalent state with the production of chromate, for a length of time such that not substantially more than 80% of the chromite is decomposed and the chromium thereof oxidized .to the hexavalent state, leaching the product of the roasting operation with an aqueous medium toproduce a solution of chromate, separating the chromate solution from the'insolubl'e material of the roasted charge, concentrating the residual insoluble mate rial to separate and recover undecomposed chromite contained therein, and thereafter roasting a charge of fresh chromium-bearingmaterial, lime, soda ash, and the, recovered undecomposed chromite while in intimate contact'with air to cause decomposition of the chromite thereof and oxidation of chromium thereof to chromate, said last-mentioned roasting operation being con ducted for a length of time such that not substantially more than 80% of the chromite is decomposed and the chromium thereof oxidized to the hexavalent state.

2. The method of producingv chromate which comprises roasting a charge of chromite-bearing material, lime and soda ash while in intimate contact with air at a temperature sufficiently high to cause decomposition of the chromite and oxidation of the chromium thereof to the hexavalent state with the production of chromate, for a length of time such that not substantially more than about to about 80% of the chromite is decomposed and the chromium thereof oxidized to the hexavalent state, leaching the product of the roasting operation with an aqueous medium to produce a solution of chromate, separating the chromate solution from the insoluble material of the roasted charge, concentrating the residual insoluble material to separate and recover undecomposed chromite contained therein, and thereafter roasting a charge of fresh chromium-bearing material, lime, soda ash, and the recovered undecomposed chromite while in intimate contact with air to cause decomposition of the chromite thereof and oxidation of chromium thereof to chromate, said last-mentioned roasting operation being conducted for a length of time such that not substantially more than 80% of the chromite is decomposed and the chromium thereof oxidized to the hexavalent state.

3. The method of producing chromate which comprises roasting a charge of chromite-bearing material, lime and soda ash while in intimate contact with air at a temperature sufficiently high to causedecomposition of the chromite and oxida- I tion of the chromium thereof to the hexavalent state with the production of chromate, for a length of time not exceeding about three hours and not longer than necessary to decompose about 80% of the chromite and to oxidizethe chromium thereof to the hexavalent state, leaching the product of the roasting operation with an aquechromium thereof to chromate, said'last-mem tioned roasting operation being conducted for a length of time such that not substantially more than 80% of the chromite is decomposed" and the chromium thereof oxidized to the hexavalent state.

4. The method of producing chromate which comprises roasting a charge of chromite-bearing material, lime and soda'ash while in intimate:

contact with air at a temperature sufficiently high to cause decomposition of the chromite and oxidation of the chromiumthereof to the hexavalent state with the production of chromate,

for a length of time such that not substantially roasting operation with an aqueous solution of sodium carbonate to produce a solution'of chromate, separating the'chromate 'solutionifrom the' insoluble materialiof the roasted charge, concen-" tratin'g the residual insoluble material to separate and recover undecomposed chromite contained therein, and thereafter roasting a charge of fresh chromium-bearing material, lime, soda ash, and

the recovered undecomposed chromite while in intimate contact with air to cause decompositionv of the chromite thereof and "oxidation of chro--. mium' thereof to chromate, said last-mentioned" roasting operation being conducted fora length of time such that not substantially more than 80% of the chromite is decomposed and the chromium-thereof oxidized to the hexavalent state.

5. The method of producing chromate which comprises roasting a chargeof chromite-bearing material, lime and soda'ash while in intimate contact with air at a temperature sufficiently high to" cause decomposition of the chromite and oxida:' tion of the chromium thereof to the hexavalent state with the production of chromate, for a length of time such that not substantially more than about 80% of the chromite is decomposedand the chromium thereof oxidized to the hexavalent state, the lime being present in the charge in amount equal to about one and one-half to about two times the weight of the chromiteof' the chromite-bearing materiaL'leaching the prod-' not of the roasting operation with an aqueous medium to produce a solution of chromate', separating the chro'mate' solution from the insoluble material of the roasted charge, concentrating the residual 'insoluble'material to separate andrecover undecomposed chromite contained therein, and thereafter roasting a charge of fresh chro mium-bearing material, lime, soda ash, and the recovered undecomposed chromite while in intimate contact with air to cause decomposition of the chromite thereof and oxidation of chromium thereof to chromate, said last-mentioned roast- .ing operation being conducted for a length of time such that not substantially more than 80% of the chromit is decomposed and the chromium thereof oxidized to the hexavalent state;

chromite and oxidation of chromium thereof to the hexavalent state with the production of chrofmate, leaching the product of the roasting opera-'- tion with an aqueous medium to produce a solution of chromate; separating the chromate solu-. tion from the insoluble material of the roasted charge, concentrating the residual insoluble material. to separate .and recover undecomposed chromite contained therein, and thereafter roast ing a charge of fresh chromium-bearing material"; lime, soda ash; and the recovered undecomposed chromite while in intimate contact with air to cause decomposition of the chromite thereof and oxidation of chromium thereof to chromate, said last-mentioned roasting operation being con-51 ducted for a length of time such that not sub-f stantially more than 80% of the chromite is decomposedand the chromium thereof oxidized to the hexavalent state.

1-7. The method of producing 'chromate which", comprises roasting, in intimate contact with air,i a charge of chromite-bearingmaterial, lime and soda1ash,in which the; soda, ash ispresent in amount not-substantially in excess of that-re quired to form sodium chromate with about of the chromium of the charge at a temperature suflicientlyhigh to cause decomposition of,

the chromite andoxidation of chromium thereof to the hexavalent, state with the production of chromate, for a length of time to cause substantially allof the soda ash to react with the chromite of the charge,- leaching the product of the roasting operation with-amaqueousmedium to produce a solution of chromate, separating thechromate solution from the insoluble material of theroastedfcharge, concentrating the residual insoluble material to separateiand recover undecomposed chromite, contained therein; and

thereafter roasting a chargeof' fresh chromium-- bearing, material, lime, "soda ash, and the recovered undecomposed chromite while in intimate contact with air to cause decomposition of the chromite thereof and oxidation of chromiumthereofto chromate, said last-mentioned roast-' ing operation being conducted for a length of time such that'not substantially more than 80% of the'chromite is decomposed andthe chromium" thereof oxidized to the hexavalent state.

8. The method ofproducing chromate which comprises roasting, in intimate contact with air, a charge of chromite-bearing material, lime andsoda ash, in which the soda ash is present in amount notsubstantially in excess 'of that required to form sodium chromate with about 70%- to about 80% of'the chromium of the charge,-

at a temperature sufficiently high to cause decomposition of the chromite and oxidation of chromium thereof to the hexavalent state with theproduction of chromataleaching the'product of the roasting operation with an aqueous medium to produce a solution of chromate, separating thechromate solution from the insoluble material ofthe roasted charge, concentrating the residual insoluble material to separate and recover un-'- decomposed chromite contained therein, and

thereafter roasting a charge of fresh chromium-- bearingmaterial, lime, soda ash, and the recovered undecomposed chromite while in intimate contact with air tocause decomposition of the chromite thereof and. oxidation of chromium thereof to chromate, said last-mentioned roasting operation being conducted for a length of time such that not-substantiallymore than '80% ofthe chromite is decomposed andthe chromium thereof oxidizedtothe hexavalent state.

QVThe methOd of producing chromate which comprises roasting, in intimate contact with air,

a charge of chromite-bearing material, lime-and soda ash, in which the soda ash is present in amountnot substantially in excess of that required to form sodium chromate, with about 80% of the chromiumof the charge and the lime is present in amountequalto about one and onehalf toabout two times the weight of the chromite. of the chromite-bearingmaterial, at a tempera-, ture suificiently high to cause decomposition of the. chromite and oxidation of the chromiumthereof to the hexavalent state with the production of chromate, leaching the product of the roastingoperation' withan aqueous medium to. producea solution of .chromate, -separating thechromate solution-from the. insoluble material, ofthe roasted charge, concentrating the residual insolublematerial to separate and-recover unde--- compose'dchr'omi-te contained therein, and thereafter roasting .a charge of fresh chromium-bear ing material, lime, soda ash, and the-recovered undecomposed chromite while in intimate contact with air to cause decomposition of the chromite thereof and oxidation of chromium thereof to chromate, said last-mentioned roasting operation being conducted for a length of time such that not substantially more than 80% of the chromite is decomposed and the chromium thereof oxidized to the hexavalent state.

10. The method of producing chromate which comprises heating a charge comprising chromium-bearing material containing ferrous chromite and lime to a temperature above 1200 C. and at least sufficiently highto cause a sintering of constituents of the charge and to Produce a product containing an altered chromite formed by substitution of the lime for ferrous oxide of the chromite of the original chromium-bearing material, roasting a charge comprising the product of said heat treatment and soda ash, while in intimate contact with air, at a temperature sufliciently high tocause decomposition of the chromate solution from the insoluble material of the roasted charge, concentrating the residual insoluble material to separate and recover undecomposed chromite contained therein, and thereafter roasting a charge of fresh chromium-bearin material, lime, soda ash, and the recovered undecomposed chromite while in intimate contact with air to cause decomposition of the chromite thereof and oxidation of chromium thereof to chromate, said last-mentioned roasting operation being conducted for a length of time such that not substantially more than 80% of the chromite is decomposediand the chromium there' of oxidized to the hexavalent state.

11. The method of producing chromate which comprises heating a charge comprising chromiumbearing material containing ferrous chromite and lime to a temperature above 1200 C. and at least sufliciently high to cause a sintering of constituents of the charge and to produce a product containing an altered chromite formed by substitution of the lime for ferrous oxide of the chromite of the original chromium-bearing material, roasting a charge comprising the product of said heat treatment and soda ash, in which the soda ash is present in amount not substantially in excess of that required" to form sodium chromate with about 70% to about 80% of the chromium of the 10 charge, while in contact with air, at a temperature sufficiently high to cause decomposition of the chromite and oxidation of the chromium thereof to the hexavalent state with the production of chromate, for length of time such that not substantially more than about of the chromite is decomposed and the chromium thereof oxidizedto the hexavalent state, leaching the product of the roasting operation with an aqueous medium to produce a solution of chromate, separating the chromate solution from the insoluble material of the roasted charge, concentrating the residual insoluble material to separate and recover undecomposed chromite contained therein, and thereafter roasting a charge of fresh chro- 'mium-bearing material, lime, soda ash, and the recovered undecomposed chromite while in intimate contact with air to cause decomposition of the chromite thereof and oxidation of chromium thereof to chromate, said last-mentioned roasting operation being conducted for a length of time such that not substantially more than 80% of the chromite is decomposed and the chromium thereof oxidized to the hexavalent state.

MARVIN J. UDY.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 219,637 Gorman Sept. 16, 1879 1,631,170 Weise June 7, 1927 1,901,939 Vett r Mar. 21, 1933 1,752,863 Tarr Apr. 1, 1930 1,948,143 Tarr Feb. 20, 1934 2,098,176 Udy Nov. 2, 1937 2,256,536 Udy Sept. 23, 1941 FOREIGN PATENTS Number Country Date 11,311 British 1891 270,143 British May 5, 1927 OTHER REFERENCES Chemical Abstracts, vol. 18 (1924), pages 2653-4, 23-56.

U. S. Bureau of Mines, by Doerner, R. I. 2999, June 1930, pages 23 and 25 (Roasting of Chromite Ores to Produce Chromates).

U. S. Bureau of Mines Bulletin V, Sept. 1939, A Study of Methods for Producing Chromates Salts from Domestic Ores, by Doerner et a1. (Copy in Div. 59.) 

